Planothidium marganaiensis sp. nov. (Bacillariophyta), a new cavum-bearing species from a karst spring in south-western Sardinia (Italy) Author Lai, Giuseppina G. 0000-0002-8506-0795 University of Sassari, Department of Architecture, Design and Urban Planning (DADU), via Piandanna 4, I- 07100 Sassari, Italy & Luxembourg Institute of Science and Technology (LIST), Environmental Research and Innovation (ERIN) Department, 41 rue du Brill, L- 4422 Belvaux, Luxembourg & laigg @ uniss. it; https: // orcid. org / 0000 - 0002 - 8506 - 0795 laigg@uniss.it Author Ector, Luc 0000-0002-4573-9445 Luxembourg Institute of Science and Technology (LIST), Environmental Research and Innovation (ERIN) Department, 41 rue du Brill, L- 4422 Belvaux, Luxembourg & luc. ector @ list. lu; https: // orcid. org / 0000 - 0002 - 4573 - 9445 luc.ector@list.lu Author Padedda, Bachisio M. 0000-0002-0988-5613 University of Sassari, Department of Architecture, Design and Urban Planning (DADU), via Piandanna 4, I- 07100 Sassari, Italy & bmpadedda @ uniss. it; https: // orcid. org / 0000 - 0002 - 0988 - 5613 bmpadedda@uniss.it Author Wetzel, Carlos E. 0000-0001-5330-0494 Luxembourg Institute of Science and Technology (LIST), Environmental Research and Innovation (ERIN) Department, 41 rue du Brill, L- 4422 Belvaux, Luxembourg & carlos. wetzel @ list. lu; https: // orcid. org / 0000 - 0001 - 5330 - 0494 carlos.wetzel@list.lu text Phytotaxa 2021 2021-03-04 489 2 140 154 http://dx.doi.org/10.11646/phytotaxa.489.2.3 journal article 4468 10.11646/phytotaxa.489.2.3 b49af0b0-2096-46b2-83f2-d83682585f31 1179-3163 5756790 Planothidium marganaiensis G.G.Lai, Ector & C.E.Wetzel sp. nov. ( Figs 2–65 ) Light microscopy ( Figs 2–57 ):— Frustules in girdle view rectangular, mostly single or forming short chains (up to two cells), clearly bent in the middle ( Figs 29 – 32 ). Valves elliptical to weakly lanceolate with convex margins and subrostrate or slightly protracted apices (never capitate). Valve dimensions (n=59): length 5.0–12.3 µm, width 2.8–4.0 µm. Rapheless valve ( Figs 2–28 ): Axial area linear, narrow and straight, sometimes lanceolate and slightly expanded in the middle of the valve. Central area asymmetrical with a unilateral small horseshoe-shaped hyaline area presenting a cavum. On the opposite side, absence of shortened striae or presence of one to two slightly shortened striae. Striae weakly radiate throughout the entire valve, 11–14 in 10 µm, measured at the central part of the valve opposite to the unilateral expansion. Raphe valve ( Figs 33–57 ): Axial area linear and narrow, slightly widened near the middle of the valve. Central area transapically rectangular to rounded, bordered by one to three shortened striae on each side, usually more distant than the others. Raphe branches straight with expanded, drop-like proximal raphe endings. Terminal raphe fissures not discernible in LM. Striae radiate throughout the entire valve, 11–14 in 10 µm. FIGURES 2–57. LM. Planothidium marganaiensis G.G.Lai, Ector & C.E.Wetzel sp. nov. Type population from the San Giovanni spring, Sardinia (Italy). Figs 2–28: Rapheless valve (SV) with cavum. Figs 29–32: Girdle views. Figs 33–57: Raphe valve (RV) with shortened striae in the central area. Scanning electron microscopy ( Figs 58–65 ):— Rapheless valve (SV) ( Figs 58–62 ): Valve surface with shallow, rounded to irregularly shaped depressions along the apical axis, mainly concentrated in the central area ( Figs 58–60 ). Externally, striae multiseriate, composed of three to five rows of small same-sized round areolae ( Figs 58–60 ). Striae continuing shortly onto the valve mantle, without interruption ( Figs 58–59 ). Internally, cavum covered by a hood with a narrow aperture towards the mantle ( Figs 61, 62 ). Raphe valve (RV) ( Figs 63–65 ): Striae composed of three to five rows of rounded areolae, not extended over the valve mantle ( Fig. 63 ). Virgae broader than striae, except near the apices. Externally, proximal raphe endings straight, terminating in expanded drop–like pores. Distal raphe fissures bent, continuing shortly onto the valve mantle ( Fig. 63 ). Internally, striae sunken between raised virgae and composed of areolae covered by individual hymenes ( Figs 64, 65 ). Central nodule raised ( Fig. 65 ). Proximal raphe endings slightly deflected into opposite sides, terminating inconspicuously ( Figs 64, 65 ). Distal raphe endings terminating on poorly developed helictoglossae, continuing shortly onto the valve mantle ( Fig. 64 ). FIGURES 58–65. SEM. Planothidium marganaiensis G.G.Lai, Ector & C.E.Wetzel sp. nov. Type population from the San Giovanni spring, Sardinia (Italy). Figs 58–62: Rapheless valve (SV). Figs 58–60: External view showing depressions along the apical axis and multiseriate striae. Note the striae continuing shortly onto the valve mantle, without interruption (Figs 58–59). Figs 61–62: Internal view showing the round cavum with a narrow aperture towards the valve mantle. Figs 63–65: Raphe valve (RV). Fig. 63: External view showing multiseriate striae not extended on the valve mantle and distal raphe ends bent unilaterally. Figs 64–65: Internal view showing striae sunken between raised virgae and composed of areolae covered by individual hymenes. Note the proximal external raphe ends slightly deflected into opposite sides and distal raphe terminating on poorly developed helictoglossae, continuing shortly onto the valve mantle. Type:— ITALY . Carbonia-Iglesias : San Giovanni cave at Domusnovas (Monte Acqua, Marganai massif), 190 m a.s.l. , 39°20’13.4’’N ; 008°37’37.1’’E , sampling dates 21 September 2016 and 31 January 2017 , collected by Giuseppina Grazia Lai (designated here, holotype : slide no. BR- 4636 Meise Botanic Garden, Belgium. Specimen here depicted in Fig. 9 ) . Etymology:— The name refers to the Marganai massif where the karst spring San Giovanni originates. Marganai (Marganái) (Iglesias)—It is an elongated mountain or a mountain of a small mountain chain: it is likely that its name is to be compared—not derived—with Lat. margo, -inis “margin” (Indo-European). Cfr. Marghine (Source: Pittau, 2013 ).